Opportunities to join ATLAS-UK arise frequently. Please refer to the following links for vacancies at particular institutes.

University of Birmingham
http://www.ep.ph.bham.ac.uk/index.php?page=exp/phdentry/index

Royal Holloway, University of London
https://www.royalholloway.ac.uk/research-and-teaching/departments-and-schools/physics/studying-here/research-degrees/

University of Warwick
https://www.findaphd.com/phds/project/phd-in-experimental-particle-physics/?p157535